Conventional X-ray sensors used to acquire an X-ray image of a person to be examined in X-ray imaging include Film/Screen systems (to be referred to as F/Ss hereinafter) which are formed by inserting a film and an intensifying screen into a cassette and Imaging Plates (to be referred to as IPs hereinafter) which are put in cassettes and used in computed radiography. X-ray sensors of these types need not be synchronized with the X-ray generation timing. A radiographer can acquire X-ray images without any blur caused by the motion of internal organs or body while observing only the breathing state and motion of the person to be examined. Hence, the X-ray generation apparatus is designed to irradiate a subject with X-rays with a delay of several ten ms to several hundred ms at the latest after the X-ray irradiation button is pressed.
In recent years, X-ray sensors capable of directly converting an X-ray image into a digital output in real time have been proposed. A solid-state photodetector can be manufactured by forming an amorphous semiconductor on, for example, a silica glass substrate and arraying solid-state photodetection elements each including a transparent conductive film and a conductive film in a matrix on the amorphous semiconductor. An X-ray detector is formed by stacking such a solid-state photodetector and a scintillator which converts X-rays into visible light.
When this X-ray detector is used, an X-ray digital image is acquired by the following process.
The X-ray detector is irradiated with X-rays that have passed through a subject. The X-rays are converted into visible light by the scintillator. The visible light is detected as an electrical signal by the photoelectric conversion unit of each solid-state photodetection element.
The electrical signal is read out from each solid-state photodetection element by a predetermined read method and A/D-converted so that an X-ray image signal is obtained.
The X-ray detector is described in detail in, for example, Japanese Patent Laid-Open No. 8-116044. There are also a number of detectors proposed, which cause a solid-state photodetector to acquire X-rays directly without using any scintillator.
Such an X-ray sensor capable of directly converting an X-ray image into a digital output in real time will be referred to as an X-ray detector hereinafter.
These X-ray detectors detect an X-ray intensity as a charge amount. To accurately accumulate the X-ray detection signal, these X-ray detectors require driving with a predetermined cycle for X-ray image acquisition, including charge removal from pixels, idling for stabilizing the potential between the pixels, charge accumulation for accumulating the X-ray detection signal, and charge read from the pixels.
The time of the charge accumulation state by the X-ray detector is limited. To irradiate the X-ray detector with X-rays in its accumulation state, the X-ray generation apparatus and the X-ray detector are synchronized with each other. More specifically, the X-ray detector is driven such that it has a plurality of driving states including charge removal from pixels, idling, and charge accumulation when the X-ray irradiation button is pressed. As soon as the X-ray detector is set in the accumulation state, an X-ray irradiation signal is transmitted to the X-ray generation apparatus to irradiate a subject with X-rays.
However, when a subject is irradiated with X-rays after driving necessary for accumulation in the X-ray detector is executed in synchronism with the X-ray irradiation button, the delay after the radiographer presses the X-ray irradiation button until actual X-ray irradiation becomes longer than a normal case without synchronization. It is therefore difficult to acquire an X-ray image without any blur caused by the motion of internal organs or body.